1. Field of the Invention
The present invention generally relates to methods for forming lithium alloys through kinetically controlled lithiation, and more particularly to a technique which simplifies lithium alloy fabrication and the control of alloy morphology through deposition on a temperature regulated substrate.
2. Discussion of the Related Art
A diversity of methods are known for combining materials in the fabrication of alloys or other devices. One of these, molecular beam epitaxy (MBE) is a process that has been applied to the fabrication of magnesium-lithium alloys. However, as is known, molecular beam epitaxy has certain shortcomings, including difficulty in implementation, expense, and time. Molecular beam epitaxy offers a means to control relatively complex fabrication processes but it is subject to rather rigid constraints (when performed properly). As a result, it is a rather expensive process to implement. In addition, growing epitaxial layers through this process is an extremely slow process. When used to combine elements such as magnesium and lithium, the MBE technique maintains relatively low concentration levels so that the magnesium and lithium constituents do not interfere with each other during the depositions process and can be combined on an appropriate substrate.
To date, there is some use of magnesium-lithium alloys in the automotive industry, and there have been suggestions of its possible use in battery technology. The rapid and controlled creation of this alloy and, especially the control of its morphology has offered a challenge to current technology. In some part, this results from the relatively high vaporization temperature of magnesium. Due to its magnesium-oxide (MgO) outer coating, it has been found that a magnesium ingot may be heated to 700 degrees C. and still remain intact.
A shortcoming of the prior art relates to the cycle life of lithium electrode batteries. It has been found that, during lithium battery recharge, the return of lithium ions to the negative battery electrode can lead to dendrite formation. This dendrite formation is believed to be the result of the inability of lithium to diffuse readily into the surface of a lithium electrode. Because of this slow diffusion into the electrode surface and bulk, the deposited lithium can grow out from the electrode as dendrites form. As the battery cycles (discharges/recharges) over time, the dendrite formation continues until one or more dendrites creates a short within an interior cell of the battery. This situation has been addressed with the creation of lithium ion batteries, however, at lessor rate capabilities and energy densities.
In prior art applications, aluminum-lithium alloys are well known. For example, the fuel tanks of the space shuttle use, or intend to use, an aluminum-lithium alloy. This alloy provides a lighter weight alternative, therefore requiring less fuel consumption. It is desirable to reuse these tanks. However, this may be problematic due to the surface heat generated upon re-entry into the earth""s atmosphere. As the tanks heat up, lithium can leach out of the aluminum. Accordingly, a technique which can potentially be used to replace the lithium lost to the surface of this and other aluminum-lithium alloys is needed.
With its light weight and optimal diffusivity, lithium incorporation can offer the potential for a wide range of improvements which include the development of new compact battery electrodes, the fabrication of new lower density alloys for the aerospace and automotive industries, the development of new selective catalysts for the oxidative coupling of methane to form C2+hydrocarbons, and the creation of pinning sites in superconductors.
Conventionally, lithium alloys have been produced using a direct-alloy method, which involves the melting and vaporizing of mole percent specific mixtures of Li and element(s), or alloy, under vacuum, and depositing the vapor on a substrate.
Accordingly, it is desirable to develop new alloy forming techniques and means to incorporate lithium using approaches which address these and other shortcomings in the prior art.
Certain advantages, and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the configurations and combinations particularly pointed out in the appended claims.
To achieve its advantages and novel features, the present invention is generally directed to a system for lithiating alloys. In accordance with one aspect of the invention, a method is provided for performing vapor deposition of a lithium alloy onto a substrate comprising the steps of vaporizing a mass of lithium at a controlled temperature while at the same time controllably heating a lithium-soluble element, such as magnesium. The method further includes the step of disposing the lithium-soluble element in the lithium vapor, wherein the lithium vapor promotes the vaporization of the lithium-soluble element to create a combined, intimately mixed vapor having both atoms and a small concentration of molecules from both the lithium and lithium-soluble element. Finally, the method includes the step of interacting the intimately mixed vapor with a temperature controlled substrate, whereby the combined vapor is deposited onto the substrate.
In accordance with the preferred embodiment, the lithium soluble element is magnesium. However, as will be appreciated, other elements may be used in accordance with the invention. For example, silicon, tin, copper, silver and zinc may each be used. In this regard, the lithium soluble element is one that, when disposed in lithium vapor, increases the escaping tendency at its surface (lowers the energy required to escape the surface) allowing atoms or molecules of the soluble element to vaporize and mix with the lithium vapor. By controlling the temperature of the lithium and the lithium soluble element, independently, the ratio of lithium to lithium soluble element in the combined vapor may be closely controlled. In this regard, it is generally preferred to control the lithium temperature within the range of approximately 350 degrees C. to 535 degrees C., and to control the magnesium (when magnesium is used as the lithium soluble element) temperature within the range of approximately 440 degrees C. to 490 degrees C. These temperature ranges can also shift slightly dependent upon the positioning of the lithium, magnesium, and substrate configurations. By way of example, when magnesium is heated to 450 degrees C. and exposed to lithium vapor (at controlled temperatures), the lithium vapor first interacts with the magnesium oxide coating, converting the surface MgO to lithium oxide plus magnesium in the initial phases. Exposure of this surface to lithium and further interaction causes the vaporization of the magnesium at temperatures far below its vaporization temperature. This allows intimate mixing of the magnesium and lithium to form the magnesium-lithium alloy. In addition, the temperature of the substrate onto which the alloy mix is deposited, may be controlled in order to control the nature of the morphology of the magnesium-lithium alloy deposited thereon.
In accordance with another aspect of the present invention, a method is provided for depositing lithium onto an aluminum element surface. This method includes the steps of vaporizing a mass of lithium and exposing the aluminum surface, at very stringent temperature, to the lithium vapor. This technique requires controllably heating an aluminum surface, so that the lithium vapor is deposited on this surface over a very precise temperature range (aluminum 570-590xc2x0 C., lithium 525 to 585xc2x0 C.) producing alloys whose lithium content does not exceed 5 wt % lithium.
In accordance with yet another aspect of the invention, an apparatus is described for performing the vapor deposition of a lithium alloy onto a substrate. In this respect, the apparatus includes several variants of lithiation containers for vaporizing and interacting a mass of lithium. These containers further include a mechanism for disposing magnesium and other lithium-soluble elements to a mass of lithium vapor. To form MgLi alloys, a heating element is placed adjacent to the magnesium source, in order to controllably heat the magnesium. Finally, a construction is provided for supporting a substrate above the container in which the lithium and the soluble element interact.
In the preferred embodiment, the apparatus further includes a temperature control mechanism configured to control the temperature of the substrate. By closely controlling the substrate temperature, the morphology of the deposited material may be closely controlled. In one embodiment, the temperature controlling device may be a cooling coil placed adjacent to the substrate surface.